Keep checking Elite for Black Ops 2 ops/challenges, enlist in them and then compete when it's time on correct gamemode, currently there's just ops/challenges for MW3(which you need premium to count) but there has been some for Black Ops 2

It should be on the console as well, but I think it's still showing coming soon, better to do it through website to ensure you can enllist when they post new ops/challenges, just keep checking as they dropped the others without warning